The reliable and effective conveyance and dispersal of fine solid particles for production of a uniform dust aerosol suspension is fraught with difficulties which may be characterized by two problem areas: (1) the feeding of bulk dust at a uniform desired rate, and (2) the dispersal of agglomerates within the bulk dust to provide a uniform dust particle size to the aerosolizing means.
The solution of these problem areas has been most successful to date by use of devices which involve the controlled abrasion of a cake of compacted powder by use of jets of compressed air or by some mechanical scraping means. One of the most acceptable devices of this category is the Wright dust feeder (see Mercer T. T., "Aerosol Technology in Hazard Evaluation," pp. 349-351, Academic Press, New York, 1973 and Wright, B. N., "Wright Dust Feed Mechanism", Journal of Scientific Instrumentation, Volume 27, pp. 12-15, 1950). In the Wright device, the dust tube contains the compressed dust or powders to be dispersed and the dust tube rotates about a scraper head piece having a slot in the head. A small amount of dust is scraped into the slot with compressed air forced through the slot, propelling dust from the slot through a passageway. The dust in the passageway is directed at high velocity against a plate which breaks up any aggregated particles and the divided dust is outputed through some nozzle means to produce a dust aerosol. However, the Wright device loses some of its reliability and effectiveness at very low powder dispersion rates, is quite complicated as well as costly, and is usually unsuitable for dusts containing a significant fraction of particles larger than 10 micrometers in diameter because of problems in forming a cohesive powdered cake of such larger particles.
A convenient means to convey solids is a rotating, spirally wound auger (see U.S. Pat. No. 3,223,227, dated Dec. 14, 1965 to Dudte). However, for the reduced feed rates of interest for dust conveyance, smaller diameter auger dimensions and closer tolerances lead to binding and plugging of passageways by the dust.
It is therefore an object of the invention to provide a device of simple construction capable of delivering a uniform and reliable quantity of dust for aerosolization.
It is a further object of the invention to provide a dust aerosol device in which the metering and conveying of dust is carried out by a tandem auger structure with the auger elements being flexible spiral wound brushes.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.